1. Field of the Invention
The present invention is directed to a method and catalyst system for producing aromatic carbonates and, more specifically, to a method and catalyst system for producing diaryl carbonates through the carbonylation of aromatic hydroxy compounds.
2. Discussion of Related Art
Aromatic carbonates find utility, inter alia, as intermediates in the preparation of polycarbonates. For example, a popular method of polycarbonate preparation is the melt transesterification of aromatic carbonates with bisphenols. This method has been shown to be environmentally superior to previously used methods which employed phosgene, a toxic gas, as a reagent and chlorinated aliphatic hydrocarbons, such as methylene chloride, as solvents.
Various methods for preparing aromatic carbonates have been previously described in the literature and/or utilized by industry. A method that has enjoyed substantial popularity in the literature involves the direct carbonylation of aromatic hydroxy compounds with carbon monoxide and oxygen. In general, practitioners have found that the carbonylation reaction requires a rather complex catalyst system. For example, in U.S. Pat. No. 4,187,242, which is assigned to the assignee of the present invention, Chalk reports that a carbonylation catalyst system should contain a Group VIII B metal, such as ruthenium, rhodium, palladium, osmium, iridium, platinum, or a complex thereof. Further refinements to the carbonylation reaction include the identification of organic co-catalysts, such as terpyridines, phenanthrolines, quinolines and isoquinolines in U.S. Pat. No. 5,284,964 and the use of certain halide compounds, such as quaternary ammonium or phosphonium halides in U.S. Pat. No. 5,399,734, both patents also being assigned to the assignee of the present invention.
The economics of the carbonylation process is strongly dependent on the number of moles of aromatic carbonate produced per mole of Group VIII B metal utilized (i.e. "catalyst turnover"). Consequently, much work has been directed to the identification of efficacious inorganic co-catalysts that increase catalyst turnover. In U.S. Pat. No. 5,231,210, which is also assigned to General Electric Company, Joyce et al. report the use of a cobalt pentadentate complex as an inorganic co-catalyst ("IOCC"). In U.S. Pat. No. 5,498,789, Takagi et al. report the use of lead as an IOCC. In U.S. Pat. No. 5,543,547, Iwane et al. report the use of trivalent cerium as an IOCC. In U.S. Pat. No. 5,726,340, Takagi et al. report the use of lead and cobalt as a binary IOCC system. In Japanese Unexamined Patent Application No. 10-316627, Yoneyama et al. report the use of manganese and the combination of manganese and lead as IOCC's.
The literature is silent, however, as to the role of the IOCC in the carbonylation reaction (i.e. the reaction mechanism). Accordingly, meaningful guidance regarding the identification of additional IOCC systems is cursory at best. Periodic table groupings have failed to provide guidance in identifying additional IOCC's. For example, U.S. Pat. No. 5,856,554 provides a general listing of possible IOCC candidates, yet further analysis has revealed that many of the members (and combinations of members) of the recited groups (i.e., Groups IV B and V B) do not catalyze the carbonylation reaction. Therefore, due to the lack of guidance in the literature, the identification of effective carbonylation catalyst systems has become a serendipitous exercise.
As the demand for high performance plastics has continued to grow, new and improved methods of providing product more economically are needed to supply the market. In this context, various processes and catalyst systems are constantly being evaluated; however, the identities of improved and/or additional effective catalyst systems for these processes continue to elude the industry. Consequently, a long, felt, yet unsatisfied need exists for new and improved methods and catalyst systems for producing aromatic carbonates and the like.